Vertical (extended) cavity surface emitting lasers (VCSEL/VECSEL) integrated with photodiodes are attractive devices for optical sensors based on self-mixing interference (SFI). These VCSEL integrated photodiodes (VIP) are the key component for the Twin-eye™ sensor from Philips Laser Sensors. In a further application of a VCSEL with integrated photodiode, the signal of the photodiode is used as a feedback signal for controlling or stabilizing the output power of the VCSEL.
Known designs of VCSEL devices with integrated photodiodes comprise an epitaxial layer stack forming the photodiode, which is arranged at the bottom of the VCSEL structure and is separated by a spacer layer from the bottom DBR (DBR: Distributed Bragg Reflector) of the VCSEL. In another design, the photodiode is integrated in the bottom DBR of the VCSEL. The latter design is disclosed for example in US 2003/0021322 A1, which describes a photodiode formed of an n-doped region, an intrinsic region and a p-doped region, which are integrated in the bottom DBR of the VCSEL. To this end, the bottom DBR is separated into a first, n-doped portion and a second, p-doped portion. The second p-doped portion forms the p-doped region of the photodiode, whereas the intrinsic region and the n-doped region are arranged between the two portions of the DBR. The VCSEL and the photodiode share a common electrode realized as an Ohmic n-contact at the n-doped region of the photodiode, which forms a spacer layer required in this design within the bottom DBR.
Although the photodiodes can be monolithically integrated with the VCSEL as shown above, the production costs are significantly higher than those of standard VCSELs. This is due to the required epitaxial layer stack which is much thicker for VCSELs with integrated photodiodes than in the case of simple VCSELs. Furthermore, in designs in which the p-doped region of the photodiode has to be electrically contacted, special fabrication steps are needed to provide an Ohmic contact to the p-doped layer of the integrated photodiode. The first drawback also applies to the device of US 2003/0021322, which requires a thick n-doped spacer layer within the bottom DBR. Such a thick spacer layer is undesirable because it increases the reactor growth time during fabrication and is almost as expensive as the simple mounting of a photodetector at the bottom side of the VCSEL instead of monolithically integrating the photodiode. The spacer layer also requires accurate thickness control during fabrication, such that mirror reflectivity and phase of the upper and lower portion of the bottom DBR match.